Radiation sensors typically convert a radiative signal into an electrical signal. An example of such a radiation sensor is a thermopile sensor which is used for infrared sensing. In a thermopile sensor a plurality of thermocouples are coupled in series or in parallel. The conversion of the radiative signal into an electrical signal is a multi-stage conversion in which not only the thermo-electrical domain but also the optical domain and the thermal domain should be considered.
With regard to the optical domain, the electromagnetic radiation in the infrared spectral range and the absorption of the radiation onto a MEMS element should be considered. With regard to the thermal domain, the heat transfer through conduction, convection and radiation should be considered. With regard to the thermo-electrical domain, the Seebeck coefficient is of importance.
Considering the different domains there are several causes for errors in the resulting electrical signal. Different external factors, apart from the radiation to be measured, contribute to the resulting electrical output signal of the sensor. Compensation therefore can be done using a reference sensor.
Given the complexity in the conversion of the signal, several external factors contribute to the electrical signal, which external factors should preferably be compensated for.
EP2172753 is a prior art solution wherein a plurality of thermopile sensors are integrated in a base wafer. A filter wafer comprises infrared filters above the individual sensors. Thus it is possible to create differing spectral transmission characteristics for the filters in front of the different thermopile sensors. One of these sensors might be a reference sensor.
Ideally the reference sensor can compensate for all influences (except for the radiation) the environment has on the measurement sensor. Careful design of the reference sensor is required. In the ideal case the transfer function of the reference sensor would be the same as the transfer function of the IR signal detector. Moreover the radiation sensor should be preferably easy to manufacture.
Therefore there is room for improving radiation sensors comprising a reference sensor for compensating for external factors in the measurement result.